1. Field of the Invention
The present invention relates to a method for punching a plurality of holes at fine pitches on brittle material such as compound sheets formed by particles and high polymers, ceramic green sheets, glass epoxy substrates, or functional particles dispersed organic films without causing any cracks, and a die to be used therefor.
2. Description of Related Art
Conventionally, brittle materials such as compound sheets formed by particles and high polymers, ceramic green sheets, glass epoxy substrates or functional particles dispersed in organic films are often used for fabricating circuit substrates, oscillating plates, or resilient plates.
For such brittle materials, a plurality of fine holes is formed at fine pitches according to the required configuration of target final products.
FIG. 4 is a cross-sectional view showing an example of the conventional punching die, wherein a die 30 comprises a plurality of punches 31, a punch plate 32, a die 34 having a plurality of fine holes 33, and a stripper 35. The reference numeral 36 denotes a cover portion and the numeral 37 denotes a brittle material such as ceramic green sheets.
However, since the demand for denser circuit boards of recent years requires lessening of the pitch of a plurality of fine holes, there is an increasing tendency to generate cracks between punched holes when the conventional die shown in FIG. 4 is used. In the case of ceramic green sheets, depending on the thickness or quality thereof, cracks occur with growing frequency during formation of a plurality of fine holes with pitches of 500 xcexcm or less.
Accordingly, it is an object of the present invention to provide a method for forming a plurality of holes of fine diameters at fine pitches on a brittle material without generating any cracks thereon.
After an elaborate study and investigation to avoid occurrence of cracks during formation of fine holes, the inventor found that the problem described above may be overcome by punching the plurality of fine holes with a time shift, rather than all at one time, and thus attained the present invention.
According to the invention, the method for punching a plurality of holes having fine diameters d at fine intervals xcex1 on a brittle material comprising the steps of forming at least two holes including a hole of fine diameter d at intervals A, each of which is at least two times the fine interval xcex1, by the use of a punching die, and punching holes between the previously punched holes at intervals A so that the distance from the punched holes of diameter d measures at least the distance corresponding to the fine interval xcex1, without removing the punches used to form the holes in the previous step.
Brittle material in the present invention includes compound sheets formed by particles and high polymers, ceramic green sheets, glass epoxy substrates, or functional particles dispersed organic films. The most preferable material is a ceramic green sheet.
The fine intervals xcex1 are preferably in the range between 30 and 500 xcexcm, and more preferably between 30 and 300 xcexcm. The fine diameter d for the hole is preferably in the range between 20 and 480 xcexcm, and more preferably between 50 and 200 xcexcm.
According to the present invention, the punching die comprising a plurality of punches having punching tips, a punch plate for holding the punch, a die having a plurality of fine holes of a prescribed diameter through which the punching tips are inserted and removed, and a stripper for grasping the tips of the punches and for peeling the punched brittle material off the punch when removing the punch from the die, wherein a plurality of punches consists of two groups of punches, so that the position of the apexes of the punching tips of one group differs from that of the other group.
While the die punch used in the invention is preferably formed in such a manner that the shortest apex-to-apex distance D out of the apex-to-apex distances between at least two groups is larger than the thickness of brittle material t, it is also possible to form so that the relation between the apex-to-apex distance D and the thickness brittle material t satisfies the relation; (2/3)xc3x97t less than D.